1. Field of the Invention
The present invention relates to a developing apparatus including a developer carrying member for carrying a developer and a detecting member for detecting a developer amount by detecting the capacitance between the developer carrying member and the detecting member. This developing apparatus can be used for an image forming apparatus, which is preferably an electrophotography apparatus such as a printer or a copying machine.
2. Description of the Related Art
There is proposed a toner remaining amount detecting method of a capacitance detecting type as a method of detecting a remaining amount of developer (hereinafter referred to as toner) stored in the developing apparatus that is used for the image forming apparatus such as the electrophotography apparatus.
For instance, Japanese Patent Application Laid-Open No. 2002-244414 discloses a developing apparatus using a contact developing method illustrated in FIG. 14, in which a developing bias power supply 105 applies an AC voltage generated by periodically turning on and off a DC bias as a developing bias to a developing roller 109 as the developer carrying member.
A voltage induced in an antenna 78 as the detecting member for detecting developer amount is measured based on an alternating electric field formed by turning on and off the developing bias, such that toner amount between the antenna 78 and the developing roller 109 can be detected. In other words, a detector 102 is used to determine whether a space between the antenna 78 and the developing roller 109 is filled with toner, or whether the toner is consumed and does not fill the space.
When the detection of the toner remaining amount is performed, it is desirable to separate the developing roller from a photosensitive drum for eliminating influence of capacitance between them. As to this apparatus, the developing apparatus can swing around a swing center 106 using a contact and separate spring 107 and a contact and separate cam 108 illustrated in FIG. 14, so the developing roller 109 having an elastic property can be made to contact with and to separate from the photosensitive drum.
On the other hand, as to a developing apparatus using jumping development, a method involving detecting the toner remaining amount by utilizing a change in capacitance is proposed, in which a developing bias that is an alternating electric field is applied to a developing sleeve as the developer carrying member.
In particular, as to a developing unit using toner that is nonmagnetic mono-component developer, it is common to provide a developing chamber with a supplying member for supplying developer to the developing sleeve. If the method of detecting the toner remaining amount through a change in capacitance is applied to a developing unit using the nonmagnetic mono-component developer, some problems will occur. For instance, since the supplying member exists, a space for housing the antenna is limited, so the capability of detecting the toner remaining amount may be deteriorated, or the toner may be blocked from being conveyed smoothly.
Therefore, as illustrated in FIG. 15 (or disclosed in Japanese Patent Application Laid-Open No. H04-234777), there is a conventional structure, in which a supplying member 80 is made up of a metal conductive support member 79 and a urethane sponge disposed on the circumference surface of the metal conductive support member 79, and an alternating electric field is applied to a sleeve 75 when the toner is supplied to the sleeve 75. Thus, a voltage corresponding to an amount of the developer is induced on the conductive support member 79, so a remaining amount of the developer can be detected based on the induced voltage.
As to this jumping development, the developing sleeve that is the developer carrying member is opposed to the photosensitive drum with a predetermined gap between them. Therefore, it is not necessary that the developing apparatus can be made contact with and separate as illustrated in FIG. 14.
Japanese Patent Application Laid-Open No. 2002-244414 discloses a structure in which the developing bias of the nonmagnetic mono-component contact developing apparatus is to be the DC bias, which is turned on and off periodically, and an alternating electric field generated in this way is used for detecting the toner remaining amount.
As for the developing apparatus using nonmagnetic mono-component developer, it is necessary to provide the developing chamber 23 with the supplying member 80. For this reason, some problems arise. For instance, a space for housing the antenna 78 is limited, so the capability of detecting the toner remaining amount may be deteriorated, or the toner may be blocked from being conveyed smoothly. In other words, it is disadvantageous to provide a special antenna 78 as a member for detecting the developer amount from a viewpoint of saving space and cost.
In addition, for a purpose of periodically turning on and off the DC bias as the developing bias without causing an image error, the developing roller is separated from the photosensitive drum during periods between printing of individual images (i.e., between so-called paper sheets) as illustrated in FIG. 14.
However, a posture of the toner existing between the developing roller and the antenna when the developing roller contacts with the photosensitive drum during an image forming period is different from a posture of the toner existing between the developing roller and the antenna when the developing roller separates from the photosensitive drum during the period between paper sheets. In this way, since an abutting and separating operation is performed with different postures of the developing apparatus, the amount of toner existing between the developing roller and the antenna changes, which causes a problem that a voltage output varies so that a certain period of time is required before the voltage output becomes stable. In this way, according to the conventional structure, the developer amount is detected with different postures of the developing apparatus. Therefore, accuracy of detection cannot be stable, making it difficult to secure correct detection.
On the other hand, as illustrated in FIG. 15, a developer supplying member is used as a member for detecting the developer amount in a non-contact developing method using the nonmagnetic mono-component developer in which the developing sleeve is separated from the photosensitive drum. This method of detecting the developer amount using the developer supplying member was applied to the contact developing apparatus.
More specifically, a developing bias having an AC component superimposed on a DC component was applied to the developing roller from a developing bias power supply 101, so as to measure a voltage that was induced on a conductive metal supporting member of the supplying member made of urethane sponge.
However, when the developing bias having an AC component superimposed on a DC component was applied to the developing roller of the contact developing apparatus using the nonmagnetic mono-component developer, smear on a white background called fog appeared. Further, when the developing roller contacts with the photosensitive drum, hitting vibration occurred between them resulting in an undesirable hitting noise.
In addition, as described above with reference to FIG. 14, if the developer amount is detected in the state where the photosensitive drum contacts with the developing roller, it was difficult to detect the developer amount accurately because of an influence of the capacitance between the photosensitive drum and the developing roller.